In the formation of micro-electronic devices using photolithographic techniques, the wavelength of the "light" utilized to form the image on the target photoresist imposes a fundamental limit on the available image definition. The image resolution, or minimum linewidth that can be imaged, is limited by the diffraction of the light at the edges of the features of the masks through which the light is projected. The commonly used figure of merit is the Fresnel number f calculated as f=W.sup.2 /G.lambda., where G is the gap distance between the mask and the target surface, .lambda. is the wavelength of the light being utilized and W is the feature size. The Fresnel number f provides a guide in assessing the obtainable resolution, with f=0.5 corresponding to about what is usually considered the resolution limit.
To allow the creation of smaller micro-electronic structures than are attainable utilizing visible or ultraviolet optical systems, X-ray sources are being utilized. Synchrotrons are particularly suitable as X-ray sources for X-ray lithography since the synchrotron provides an intense, steady beam of substantially collimated X-ray photons having a mixture of wavelengths spanning soft to hard X-rays. Because the wavelength .lambda. of X-rays is smaller than the wavelength of optical or ultraviolet light, X-ray lithography inherently allows smaller features to be created. However, the feature size for X-rays is also ultimately limited, as the Fresnel number criterion also applies to X-ray lithography systems. The formula for the Fresnel number is approximate since it does not include physical effects, and resolutions in X-ray lithography systems less than 1,000 Angstroms (A) have been demonstrated with gaps larger than those that might be expected from the Fresnel number calculated for such systems. Nonetheless, the Fresnel number provides an approximate criterion for determining the ultimate resolution. Using this criterion, for example, it is found that to image 0.25 micrometer (.mu.m) lines with one nanometer (nm) radiation would allow a maximum gap of only 12.5 .mu.m. Thus, as the required line resolution shrinks, so does the available working distance between the mask and the target surface. It is generally considered difficult to perform X-ray lithography exposures at distances between the mask and target of less than 10 .mu.m. It may be noted that there are two types of images that can be considered in determining the resolution in X-ray lithography, the aerial image (the X-ray intensity at the target surface) and the latent image (the image recorded in the target photoresist resist material).
Phase shifting masks have been used to increase the image definition in projection optical systems, and their use has been proposed to allow extension of the resolution limit of conventional visible and ultra-violet light optical lithographic systems. The phase shift mask includes a transparent layer of suitable thickness defining certain features which introduces a half wavelength (.pi.) phase shift of the field E.sub.1 of the light transmitted through the layer relative to the field E.sub.2 of the light transmitted through an area without the transparent layer. The total field E.sub.t is obtained by addition of the two fields, i.e., E.sub.t =E.sub.1 +E.sub.2, so that at some position along the image plane, the total field must become zero because of the continuity requirement. This creates a sharp modulation in the intensity pattern. A judicious choice of the phase shifting overlayer can improve the image even for complex patterns, although the technique works best for regular and repetitive cases such as those used in the manufacture of dynamic random access memories (DRAMs).
It has been suggested that an X-ray mask having an absorbent thickness appropriate for yielding a .pi. phase shift can improve image sharpness. See Y. C. Ku, et al., "Use of a Pi-Phase Shifting X-Ray Mask to Increase the Intensity Slope at Feature Edges," J. Vac. Sci. Technol. B, Vol. 6, No. 1, January/February 1988, p. 150-153. The masks described therein are absorbing masks, and the phase effects were used to refine the image rather than to define it.